Not applicable.
Not applicable.
The present invention is directed to processes for producing zirconium basic sulfate. More particularly, the present invention is directed to processes for producing zirconium basic sulfate from a zirconium oxychloride solution. The present invention is additionally directed to materials that include zirconium basic sulfate. The invention finds application in, for example, the production of zirconium basic sulfate from zirconium metal, zirconium scrap, zirconium-containing ores, and other materials including zirconium in metallic or other forms.
Zirconium sulfate, also called zirconium basic sulfate or xe2x80x9cZBSxe2x80x9d, is a key intermediate in the manufacture of zirconium chemicals from ores. It is currently a multi-million kilogram per year industry. The ores that are processed into ZBS are chiefly zircon, ZrSiO4, and to a much lesser extent, baddeleyite, which is impure ZrO2. The two most common means of opening the ores are caustic fusion and carbochlorination. These means are well known and are generally described in, for example, Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 24, Third Edition, 1984, p. 863, the substance of which is hereby incorporated herein by reference. Carbochlorination utilizes zirconium tetrachloride as a source of zirconium ions in producing zirconium sulfate, but that process is disadvantaged, in terms of yield, relative to the caustic fusion process.
In the caustic fusion process, zircon ore is fused with sodium hydroxide, or in variations, with sodium carbonate or lime. The product of the fusion is called a xe2x80x9cfritxe2x80x9d and is contacted with water to dissolve and remove the silicon moiety as, for example, sodium silicate in the case where the ore is fused using sodium hydroxide. The leached frit is then dissolved in hydrochloric acid to yield a zirconium oxychloride solution. Because the stoichiometric ratio of zirconium oxychloride is two moles of hydrochloric acid for every mole of zirconium, there is little point in adding more hydrochloric acid than is necessary to dissolve the washed frit. Formation of zirconium oxychloride solution in this way is described in, for example, Beyer, et al., Ames Laboratory Report ISC 437 (Dec. 28, 1953), the substance of which is hereby incorporated herein by reference. A method for preparing a commercial ZBS from the zirconium oxychloride solution is described in, for example, U.S. Pat. No. 1,316,107, which also is incorporated herein by reference. A source of sulfate ions, such as sulfuric acid, is added to the zirconium oxychloride solution, and the mixture is then heated with stirring until a precipitate of ZBS forms. The mixture is allowed to digest for a period of time to allow substantially complete precipitation of ZBS as fine particles in the reaction vessel. The warm reaction mixture is filtered and washed to give a moist paste or cake of ZBS which is substantially free of all the metallic impurities that occur in the zirconium-containing ore. The filtrate contains zirconium ions that do not react to form product and, hence, are a yield loss.
In the alternate process for ZBS production, carbochlorination, zirconium values are typically first converted to zirconium tetrachloride by the high-temperature reaction of a mixture of zirconium-containing ore, a carbon source such as coke, and a chlorinating agent such as chlorine. In the case of zircon, this reaction produces substantially equimolar quantities of zirconium tetrachloride and silicon tetrachloride, ZrCl4 and SiCl4, respectively, as well as chlorides of many of the impurities present in the ore. Because silicon tetrachloride is a valuable product useful in, for example, producing fiber optic silica, carbochlorination makes use of the zirconium-containing ore""s silicon content, which is merely discarded as a component of a waste sludge in the caustic fusion process. Zirconium tetrachloride is a fuming subliming solid and reacts vigorously and irreversibly with water to form a zirconium oxychloride solution. The zirconium oxychloride solution derived from the zirconium tetrachloride may then be reacted with sulfuric acid, as described above in connection with the caustic fusion process, to produce ZBS.
The amount of hydrochloric acid used to dissolve the leached frit in the caustic fusion process is the minimum amount required to allow the dissolution to occur in a convenient time period. In practice, this amount is that which meets any unneutralized caustic plus the stoichiometric ratio of 2HCl:1Zr, plus a further increment so that the dissolution will be suitably rapid. Ordinarily, this results in a HCl:Zr ratio somewhat larger than 2. Thus, zirconium oxychloride-containing solutions made for ZBS preparation via the caustic fusion process will have an acidity somewhat above 2 moles/liter of hydrogen ion concentration per mole/liter of zirconium. By contrast, zirconium oxychloride-containing solutions prepared from zirconium tetrachloride are found to have a HCl:Zr ratio of about 3.5 to 3.8, with a correspondingly higher acidity. When the higher acidity solutions are converted to ZBS (all other conditions of metal loading, sulfate:Zr ratio, reaction time, and temperature being kept constant), a lower yield of ZBS is obtained. Because zirconium ions lost to the filtrate during precipitation of ZBS are relatively dilute in a large volume of acid, they are not economically recoverable, and end up as a valueless sludge. Thus, the filtrate left upon separating the ZBS precipitate in carbochlorination contains a much higher quantity of unreacted zirconium ions than in the caustic fusion process and, consequently, the yield of ZBS is much lower in the carbochlorination process than in the caustic fusion process.
The carbochlorination process is well known and is disclosed in, for example, U.S. Pat. No. 1,376,161, the entire disclosure of which is hereby incorporated herein by reference. The ""161 patent does not refer to the yield deficiency of that method. The caustic fusion process, on the other hand, although providing a higher yield of ZBS, does not provide a means to recover the silicon values from zirconium-containing ore, as in the carbochlorination process.
An alternative to the caustic fusion and carbochlorination processes is to react materials containing metallic zirconium with chlorine at high temperatures to recover zirconium from the materials as zirconium tetrachloride. A carbon source is not necessary in that process. The process, however, requires a starting material composed of or including zirconium metal, which excludes the use of, for example, zircon and baddeleyite ores. In addition, chlorination of metallic zirconium is highly exothermic and difficult to control.
Various means have been attempted to raise the yield of ZBS from zirconium tetrachloride. For example, it is known that it is possible to recoup lost yield from a zirconium tetrachloride solution by neutralizing some excess acidity. This may be accomplished by adding any of the basic reagents sodium hydroxide, sodium carbonate, or ammonia to the sulfate reaction mix. Adding such reagents increases cost and introduces undesirable foreign positive ions, such as sodium or ammonium, into the reaction mix. For most downstream applications of ZBS, the amounts of foreign positive ions retained in the ZBS cake are unacceptable. Furthermore, precipitations conducted under these more basic conditions risk co-precipitating metallic impurity ions, such as iron or aluminum ions.
It is also known in the art to increase yield in the ZBS precipitation step by forcing the precipitation or by lowering the mole ratio of sulfate to zirconium. For example, if ZBS is first made using a ratio of sulfate to zirconium of 3:5, a reaction similar in all other respects except using a ratio of 2:5 will provide a higher ZBS yield. It also is known to increase ZBS precipitation yield by adding a large volume of cold water in the latter stages of the precipitation reaction. However, products produced by modifying the ZBS precipitation reaction in these ways are often unreactive in downstream reactions. As used herein, the xe2x80x9creactivityxe2x80x9d of a ZBS precipitate refers to the ability of the precipitate to dissolve rapidly and completely under mild conditions to give a clear solution. Only a small population of unreactive particles in the precipitate will result in a turbid solution, which may be unacceptable for subsequent reaction steps. The reasons for the unreactive nature of such ZBS precipitation products are not well understood, but it is theorized that the ZBS particle surface functionalization or particle morphology may be adversely affected by the steps taken to modify the precipitation reaction.
Thus, several drawbacks are associated with known processes for forming ZBS from zirconium oxychloride solutions and from zirconium metal and zirconium-containing materials.
Accordingly, the need exists for a process for providing ZBS from zirconium metal and zirconium-containing materials which improves upon the yield of the above conventional processes, while also providing a ZBS product that will retain reactivity for use in subsequent reactions.
The need also exists for a process for providing ZBS from zirconium metal and zirconium-containing materials and by which the zirconium values of a zirconium oxychloride solution may be captured in high yield and with high purity.
In addition, the need exists for a process for providing ZBS from zirconium oxychloride solutions, zirconium metal, and zirconium-containing materials in high yields and without need for the addition of sodium hydroxide, sodium carbonate, ammonia, or other chemicals which may adversely affect the purity of the ZBS product.
In order to address the above-described needs, the present invention is directed to a process for providing a ZBS wherein a zirconium oxychloride solution is provided and is then dialyzed against a liquid selected from water and an aqueous solution across at least one anion exchange membrane. The dialysis produces a dialyzate and diffusate, which may be separately collected. The dialyzate retains at least 90 percent of the zirconium ions of the zirconium oxychloride solution and is at a lower total acidity than the zirconium oxychloride solution. A precipitate that includes ZBS may be formed from at least a portion of the diffusate produced in the dialysis.
The zirconium oxychloride solution may be provided by any known process. In one embodiment of the process of the present invention, the zirconium oxychloride solution is provided by first providing a solid zirconium tetrachloride and then dissolving the solid zirconium tetrachloride in one of water and a hydrochloric acid solution. The solid zirconium tetrachloride may be obtained as a reagent or provided by any known process such as, for example, chlorinating zirconium metal or a zirconium-containing material to provide a chlorinated material that includes zirconium tetrachloride. The zirconium-containing material may be any material suitable for the formation of ZBS such as, for example, zircon, baddeleyite, oxide materials including zirconium, and materials including zirconium metal.
The process of the present invention also is directed to producing ZBS by recovering a solid zirconium tetrachloride from zirconium metal or a zirconium-containing material, and then dissolving the solid zirconium tetrachloride in one of water and a hydrochloric acid solution to provide a zirconium oxychloride solution. The zirconium oxychloride solution is dialyzed against water across an anion exchange membrane to provide a dialyzate and a diffusate. The dialyzate includes at least 90 percent of the zirconium ions of the dialyzed zirconium oxychloride solution, and the total acidity of the diffusate is greater than the water used in the dialysis. At least a portion of the dialyzate is combined with one of sulfuric acid and a sulfuric acid solution, and the resulting mixture is heated to form a precipitate that includes solid ZBS.
The present invention also is directed to a material composed wholly or partially of ZBS and which is formed by a process of the present invention.
As used in the present description of the invention, the terms xe2x80x9csolution, xe2x80x9cwaterxe2x80x9d, and xe2x80x9ctotal acidityxe2x80x9d have the following meaning. The term xe2x80x9csolutionxe2x80x9d is used herein to refer to, for example, zirconium oxychloride solution, hydrochloric acid solution, and sulfuric acid solution. In that context, xe2x80x9csolutionxe2x80x9d refers to pure forms of those solutions, as well as forms including acceptable minor levels of solids and other solutes solvents, whether intended or present as impurities. For example, iron, aluminum, and titanium are commonly present as impurities in zirconium oxychloride solutions, and minor amounts of those impurities do not significantly affect the quality of ZBS that is produced from the solutions. The term xe2x80x9cwaterxe2x80x9d refers to pure water or water including acceptable minor amounts of dissolved and solid impurities. It is contemplated, for example, that in conducting the process of the invention waste water from a tank used in the process, or recycled water from another step in the process, may be used and may include insignificant amounts of impurities. The term xe2x80x9ctotal acidityxe2x80x9d, also referred to herein as xe2x80x9cTAxe2x80x9d, refers to the concentration of hydrogen ions and acidic anions in a solution, and is determined by titrating the solution with sodium hydroxide to the phenophthalein end point. Thus, total acidity typically is expressed in units of moles per liter.
The process of the present invention allows for reduction in the total acidity of a zirconium oxychloride solution without a significant reduction in the zirconium content of the solution. For example, the inventor has determined that a process carried out according to the invention can result in less than 1 percent of the zirconium ions being lost to the diffusate during the dialysis using a conventional anion exchange membrane. The reduction in the total acidity of the zirconium oxychloride solution allows for the formation of ZBS with high yield. The dialysis also may reduce the content of ions of other metals within the zirconium oxychloride solution, enhancing the purity of the ZBS. Because the process of the invention does not require the addition of reagents to the zirconium oxychloride solution to adjust the total acidity of the solution or to force the equilibrium of the ZBS precipitation reaction, no impurities are introduced into the ZBS product.
These and other advantages of the process and product of the present invention will be apparent on consideration of the following detailed description of embodiments of the invention.